It’s been a long time coming: a cancer drug that slays disease based not on the organ where it originated, but on its DNA. In May, the U.S. Food and Drug Administration (FDA) greenlighted the first such treatment, called pembrolizumab. Manufactured by Merck and branded Keytruda, the drug, which had already been approved to treat melanoma and a handful of other tumor types, can now be prescribed for any advanced solid tumor in children or adults, on one condition: The cancer cells must carry a defect that goes by the awkward name of “mismatch repair deficiency.” This means that whether the cells turned cancerous in the pancreas, the colon, the thyroid, or any one of a dozen other tissues, they are riddled with mutations in genes that repair DNA.
The FDA approval signifies a big shift for the field. That’s because tumors that arise in different organs may have more in common than those growing in the same place—but turning that knowledge into treatments hasn’t been easy. A breakthrough came in 2015, when doctors at Johns Hopkins University in Baltimore, Maryland, led by Luis Diaz (now at Memorial Sloan Kettering Cancer Center in New York City), studied pembrolizumab in colon cancer patients. They noticed something striking: Tumors in eight out of 13 people with mismatch repair deficiency shrank and those in four others remained stable on the drug, an “immune checkpoint inhibitor” that revs up the immune system to fight cancer. Twenty-five other colon cancer patients without the defect didn’t respond to treatment. Doctors theorized that because cells with the deficiency accumulate hundreds of mutations, the immune system more easily recognizes the diseased cells as foreign, and kills them.
A study published in June by Diaz, Dung Le at Johns Hopkins, and many others expanded testing to 86 seriously ill patients with 12 different cancers, all of which had mismatch repair deficiency. Fifty-three percent responded to the drug. Based partly on this work, FDA gave its stamp of approval—one that oncologists hope will be the first of many for this cancer-fighting strategy.
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